During the lifetime of a patient, it may be necessary to perform a joint replacement procedure on the patient as a result of, for example, disease or trauma. The joint replacement procedure, or joint arthroplasty, may involve the use of a prosthesis which is implanted into one of the patient's bones. In the case of a hip replacement procedure, a femoral prosthesis is implanted into the patient's thigh bone or femur. One type of early femoral prosthesis was typically constructed as a one-piece structure having an upper portion which includes a spherically-shaped head which bears against the patient's pelvis or acetabulum, along with an elongated intramedullary stem which is utilized to secure the femoral component to the patient's femur. In order to secure the prosthesis to the patient's femur, the medullary canal of the patient's femur is first surgically prepared (e.g. reamed and/or broached) such that the intramedullary stem of the femoral prosthesis may be subsequently implanted therein. The femoral prosthesis may be press fit into the medullary canal or, in the alternative, bone cement may be utilized to secure the femoral prosthesis within the medullary canal.
During performance of a joint replacement procedure, it is generally important to provide the orthopaedic surgeon with a certain degree of flexibility in the selection of a prosthesis. In particular, the anatomy of the bone into which the prosthesis is to be implanted may vary somewhat from patient to patient. For example, in the case of a femoral prosthesis, the patient's femur may be relatively long or relatively short thereby requiring use of a femoral prosthesis which includes a stem that is relatively long or short, respectively. Moreover, in certain cases, such as when use of a relatively long stem length is required, the stem must also be bowed in order to conform to the anatomy of the patient's femur.
Such a need for prostheses of varying shapes and sizes can create a number of problems in regard to use of a one-piece prosthesis. For example, a hospital or surgery center must maintain a relatively large inventory of prostheses in order to have the requisite mix of prostheses needed for certain situations such as trauma situations and revision surgery. Moreover, since the bow of the stem must conform to the bow of the intramedullary canal of the patient's femur, rotational positioning of the upper portion (i.e. proximal end) of the prosthesis is limited thereby rendering precise locating of the upper portion and hence the head of the prosthesis very difficult. In addition, since corresponding bones of the left and right side of a patient's anatomy (e.g. left and right femur) may bow in opposite directions, it is necessary to produce “left” and “right” variations of the prosthesis in order to provide anteversion of the bowed stem thereby further increasing the inventory of prostheses which must be maintained.
As a result of these and other drawbacks, a number of modular prostheses have been designed. As its name implies, a modular prosthesis is constructed in modular form so that the individual elements or features of the prosthesis can be selected to fit the needs of a given patient's anatomy. For example, modular prostheses have been designed which include a proximal neck component which can be assembled to any one of numerous distal stem components in order to create an assembly which fits the needs of a given patient's anatomy. Such a design allows the distal stem component to be selected and thereafter implanted in the patient's bone in a position which conforms to the patient's anatomy while also allowing for a limited degree of independent positioning of the proximal neck component relative to the patient's pelvis.
In another type of modular implant, three components (in addition to the head) are utilized: a distal stem component that is engaged within the femur, a proximal metaphyseal filling component, and an intermediate neck component that supports the head component on the distal stem component. The provision of three components has greatly increased the degree of flexibility in producing a total hip implant that most closely approximates the patient's skeletal anatomy and normal joint movement. One such system is the S-ROM® total hip system marketed by DePuy Orthopaedics, Inc. The S-ROM® total hip system offers neck and head components having different lengths, different lateral offsets of the neck relative to the stem, as well as different stem configurations.
In order to properly size the final implant, many systems utilize trial implants, commonly referred to as simply trials. Thus, in modular systems such as the S-ROM® instrument system, neck trials, proximal body trials, distal stem trials, head trials and sleeve trials can be provided. Each trial is provided in a number of sizes and geometries to give the surgeon a wide range of combination from which to choose. The trials afford the orthopaedic surgeon the opportunity to assess the fit and position of a final implant without having to complete the fixation. Like the implant systems itself, the trials are modular to reduce the inventory of components and the complexity of the trialing process.
Success of the hip replacement procedure depends in large part on the technical precision with which the final implant is inserted and the modular components oriented relative to each other. Current trialing systems have performed well in assessing implant size and gross orientation, relying primarily on laser marking, for example. However, there remains an unfulfilled need for a trialing system (as well as a modular implant system) that is able to accurately reproduce the anteversion angle of the femur. The anteversion angle is an angle of rotation between the ball end of the femur and the plane of the intramedullary canal of the bone. In the context of the modular implant, the anteversion angle is the relative angular rotation of the proximal neck component relative to the distal stem component. Proper rotational position, or anteversion angle, allows for accurate and stable reproduction of the mechanical orientation and function of the reconstructed hip joint.
For implants having a straight distal stem, the proper anteversion angle can be obtained by simply spinning the distal stem within the prepared bore in the femur. In a typical case, the surgeon can visually evaluate the orientation of the proximal body relative to the surrounding anatomy. If a trial is used the trial is removed and the final prosthesis is implanted as close to the trial position as possible. Sometimes, x-rays are used to verify the rotational alignment, while some systems rely upon external references to verify alignment.
The S-ROM® total hip system described above utilizes laser markings on the proximal end of the distal stem and on the proximal sleeve. These markings enable the surgeon to measure relative anteversion rotational alignment between the components. Since the sleeve has infinite anteversion it is not necessarily oriented relative to a bony landmark that can be used to define the anteversion angle. In fact, for simplicity, most current sleeves are oriented with the laser marking pointing directly laterally into the remaining available bone of the femur.
The problem of ensuring proper anteversion alignment is exacerbated where the modular system includes a curved distal stem. As explained above, where a long distal stem is utilized, it must be curved to follow the natural shape of the femur. Obviously, a curved stem means that the rotational alignment cannot be modified by simply rotating the stem within the prepared bore, since the bore itself is also curved. Monolithic, or one-piece stems must be offered with a predetermined anteversion angle, typically fifteen degrees. At present no modular implant or modular trial system is available that allows for accurate reproduction of the anteversion rotational orientation.